noun, feminine/ɛs.pɛs sɔ.nɑ̃t.z‿e tʁe.by.ʃɑ̃t/plural: p
cash, spot cash, hard cash
“payer en espèces sonnantes et trébuchantes” — to pay in coin of the realm
From the fact that a real coin was supposed to emit a certain sound, and have a sufficient weight to tip the scales (i.e. the trébuchet).